Nitric oxide (NO) or an oxidised form (N2O3, produced by reaction of NO with molecular oxygen) readily nitrosates thiols and secondary amines under physiological conditions. Inorganic phosphate is an effective inhibitor. It is possible that phosphate acts catalytically to hydrolyze N2O3 by forming a nitrosated phosphate that undergoes P-O bond cleavage to yield metaphosphate that in turn is hydrated to orthophosphate. To test this possibility nitric oxide was generated in the presence of 18O phosphate see if NO/N2O3 catalyzed a loss of 18O label compared with an untreated phosphate sample. NO/N2O3 induced very little loss of label indicating that at most only 10% of the N2O3 formed was hydrolyzed to nitrite by way of a labile nitrosated phosphate and other mechanisms must be sought for phosphate inhibition of S-nitrosation.